It's *not* as advertised, that's the point.
If a high street shop said "Up to 50% off" their goods, and the maximum discount of anything there was 5%, Trading Standards would be fining them for misleading the consumers.
There are specific rules about "Up to..." discounts that specify the minimum set of products that have to be available at the maximum discount, for both the 'number of line items' and the 'minimum stock availability' at the start of the sale period.
Secondly, the Weights and Measures Act says that it is illegal to sell a customer a "1 Litre" bottle of water that only contains 900ml, but it's fine to sell it if it's got more than 1L inside.
So manufacturers actually slightly overfill everything to make sure that they don't *ever* under-fill, and spend a lot of money on very accurate and precise measuring devices to minimise the over-filling margin needed. (We sell these weighing devices, it's stunning how fast they can pay for themselves!)
So why is it only ISPs that are permitted to advertise and sell a service that they *cannot provide to anyone*?
Would you be happy if you paid £10 for an "Up to 10Mb" service that gave you 1Mb, and then 'upgraded' to paying £30 for an "Up to 20Mb" service where you still got 1Mb?
By your reckoning, that would be absolutely fine.
One earlier poster suggested that the ISPs should be banned from advertising "Up to x" bandwidth, and instead could only advertise "At least x". I could get behind that.
That would mean that people living a long way from the exchange/poor exchanges could not be sold the 'fastest' product.
That way you'd be able to buy a 5Mb connection knowing that you'll get 5Mb. Of course the ISP will probably throttle your connection down to that 5Mb if your line is actually capable of more bandwidth, but you won't mind that throttling because you're still getting what you paid for.